1. Field of the Invention
The disclosure relates to a water electrolysis system and a method of stopping an operation of the water electrolysis system.
2. Discussion of the Background
For example, a solid polymer type fuel cells generate DC electric energy when anodes thereof are supplied with a fuel gas, i.e., a gas mainly composed of hydrogen, e.g., a hydrogen gas, and cathodes thereof are supplied with an oxygen-containing gas, a gas mainly composed of oxygen, e.g., air.
In general, a water electrolysis apparatus is used to produce a hydrogen gas as a fuel gas. This water electrolysis apparatus electrolyzes water to generate hydrogen (and oxygen), and uses a solid polymer electrolytic membrane (ion exchange membrane) for the electrolysis. Electrode catalytic layers are respectively provided on both sides of the solid polymer electrolytic membrane to form an electrolytic membrane/electrode assembly. Power feeders are respectively provided on both sides of the electrolytic membrane/electrode assembly to form a unit.
With a plurality of such units stacked, a voltage is applied to both ends of the stack of units in the laminating direction, and water is supplied to the anode side. As a result, water is decomposed to produce hydrogen ions (protons). The hydrogen ions permeate through the solid polymer electrolytic membrane to the cathode side, and are bonded with electrons to produce hydrogen. On the anode side, oxygen produced together with the hydrogen ions are discharged with excessive water from the units.
This type of water electrolysis apparatus uses a high pressure hydrogen producing apparatus which generates high pressure hydrogen (generally of 1 MPa or higher) on the cathode side. This high pressure hydrogen producing apparatus has high pressure hydrogen filed in the fluid passage of the cathode-side separator with a solid polymer electrolytic membrane disposed in between, and has water and oxygen of normal pressure present in the fluid passage of the anode-side separator. At the time of stopping the operation of the water electrolysis apparatus (stopping supplying generated hydrogen), therefore, it is necessary to eliminate the pressure difference between both sides of the solid polymer electrolytic membrane to protect the solid polymer electrolytic membrane.
Accordingly, normally, a process of forcibly lowering the pressure of hydrogen filled in the fluid passage of the cathode-side separator to near the normal pressure is carried out after the water electrolysis process is stopped by stopping supplying power to the individual power feeders.
If reduction of the hydrogen pressure takes place rapidly at that time, the hydrogen gas staying inside the solid polymer electrolytic membrane expands, which may produce blisters. It is therefore necessary to gradually reduce the hydrogen pressure. This takes a considerable time for the hydrogen pressure in the fluid passage of the cathode-side separator to become the normal pressure after the water electrolysis process is stopped, so that hydrogen is likely to permeate (cross leakage) to the anode side from the cathode side during the pressure reduction. This brings about a problem that the anode catalyst is reduced by hydrogen, thus degrading the water electrolysis performance.
As a solution to this problem, an operation stopping method for a water electrolysis system which is disclosed in Japanese Unexamined Patent Application Publication No. 2010-236089 includes a step of applying a voltage after supply of hydrogen from an electrolysis chamber on the cathode side is stopped, and a step of reducing the pressure at least in the cathode-side electrolysis chamber with the voltage applied.
This operation stopping method permits hydrogen leaked to the anode side from the cathode side to be protonated again with the voltage applied, and the protonated hydrogen is allowed to permeate through the electrolytic membrane to return to the cathode side due to the membrane pump effect. This makes it possible to suppress residence of high pressure hydrogen and thus inhibit reduction (deterioration) of the catalyst electrode with hydrogen, which would otherwise degrade the water electrolysis performance.